On 16 Jul 2014 it will be 20 years since the first of some 21 discernible fragments of Comet P/Shoemaker-Levy 9 collided with Jupiter, and the bombardment continued to Jul 22.
The 1994 spectacular was the first collision of two solar system bodies ever observed. Some eight spacecraft and 21 observatories on Earth had eyes on the bombardment, and hundreds of thousands of Earthlings viewed it in near-real time via the National Space Science Data Center. The 15th anniversary of the bombardment coincided with another strike on Jupiter. On 21 Jul 2009, a comet or asteroid punched an even bigger hole in the planet -- Earth-sized, by some accounts -- yet scarcely caused a stir on Earth because the event had almost no audience. One news organization saw the 2009 strike as a chilling example of the "terrible randomness" of Space. The twin anniversaries in Jul 2014 and the meteor strike in Chelyabinsk, Russia, in Feb 2013 underline the observation.
Date written/update: 2013-11-20